What happened
A float-equipped de Havilland DHC-6 Twin Otter departed Langara for Alliford Bay in the Queen Charlotte Islands with eight passengers and two pilots. During the flight, the pilot noticed a low-level caution light for the forward fuel tank, despite the gauge indicating 310 pounds of fuel remaining. The pilot elected to continue the flight, assuming gravity feed would maintain engine operation. Approximately five minutes later, the number 2 engine failed following a low fuel pressure warning, and the fuel gauge showed approximately 200 pounds.
Near Yakoun Lake, the pilot decided to perform an emergency landing. While the aircraft landed safely, the number 1 engine also stopped while attempting to taxi to the beach. The fuel tanks were found to be empty. The passengers were later transported to their destination, and the aircraft was refuelled and returned to service.
The investigation
The investigation examined the aircraft's fuel systems, the accuracy of the fuel gauges, and the flight planning records. Investigators found that the forward fuel gauge was inaccurate, reading significantly lower than the actual fuel volume at low levels. Additionally, the aft fuel tank gauge was completely unserviceable.
Records in the journey logbook were found to be inconsistent with actual fuel consumption and the pilot's statements. While the pilots had performed morning fuel dips, the investigation determined that the dipstick method is only an approximate measurement and can be prone to error. Furthermore, the investigation looked into the operator's Minimum Equipment List (MEL) and how it was applied to the unserviceable gauge.
Findings
- The aircraft experienced fuel exhaustion because the crew failed to establish the actual fuel quantity on board before or during the flight.
- The forward fuel gauge provided inaccurate readings, and the aft tank gauge was inoperative.
- The crew did not follow the required MEL procedures for operating with an unserviceable fuel gauge, which would have required filling the tanks to capacity or performing more frequent, accurate checks.
- The pilots did not accurately monitor fuel burn during the flight, particularly as the flight duration exceeded the original flight plan due to weather.
- The captain relied on the inaccurate forward gauge and the approximate dipstick readings to determine fuel loads.
- The decision to land at Yakoun Lake after the first engine failure likely prevented a more serious accident.